Vol.2, Issue.2, Mar-Apr 2012 pp-303-309 ISSN: 2249-6645 A NEW CIRCUIT TOPOLOGY FOR OPEN CIRCUIT AND SHORT CIRCUIT FAULT TOLERANT DC-DC CONVERTER P. KRISHNA CHAND 1, P.SIVA SANKAR 2 *(Student, Department of Electrical and Electronics Engineering, KL University) India ** (Assistant Professor, Department of Electrical and Electronics Engineering, KL University) India ABSTRACT This paper describes a new design for a fault tolerant H- bridge dc-dc converter. Open circuit and short circuit fault tolerance is achieved using multilevel converter topology in combination with pulse width modulation control strategy allowing a large set of converter to produce bidirectional flows at any required output voltage. If two switches fail at particular instant then also fault tolerant can be achieved. Fault tolerant ability of proposed converter to recover the required output voltage is verified by computer simulation using MATLAB/SIMULINK with 1kw resistive load. Keywords- DC-DC conversion, fault tolerant, multilevel system. I. INTRODUCTION DC-DC converters are commonly used in wide variety of applications, including a number of critical applications in which very high levels of reliability are required because the loss of converter operation can have serious consequences. For example, control of car is lost when the supply voltage for a brake-by-wire system has collapsed due to converter failure. Another critical application is the use of dc-dc converter in low- refrigeration application developed for use in an ambulance to maintain saline temperature within a specific range for immediate injection into a patient [1]. In such an application, the loss of control of the converter voltage can lead to a temperature difference of several degrees and serious medical complications. pseudo fault tolerant modular multilevel dc-dc converter [9], which could continue to operate in the event of a short circuit fault in any of the series connected modules the circuit however, could not operate successfully if one of its devices had experienced an open circuit fault, as recognized by the authors. Ceglia et al. [11] developed a circuit in that circuit, as proposed by Ceglia et al., suffers from a number of potential problems and drawbacks when operated as a dc-dc converters including high operational losses and long term reliability problems, as some of the switches are required to conduct permanently. In this paper, a new pulse width modulation (PWM) control strategy is developed and applied to modified circuit topology, in which the original converter is extended by the addition of an extra switching leg and bidirectional selector switches, to overcome these problems. If fault occur in an extra switching leg and also in the converter switches we can add one more leg to overcome these problems. In this paper the proposed convert has two auxiliary legs and selector cells so it can be called as HBALSC (H-bridge with auxiliary leg and selector cells). In order to achieve highly reliable dc-dc conversion systems, N+M redundancy concepts have been proposed in the past [2], [3]. This is costly option in which one or more additional dc-dc converters are connected in parallel to achieve the required levels of redundancy in case of failure of the main converter. More recently, it has been shown that multilevel dc-ac converter topologies can be operated as fault tolerant circuits [4]-[6]. Multilevel dc-dc converters with multiple dc sources and no magnetic storage components have been proposed recently to achieve variable dc output voltage operation [7]. Initial investigations of the multilevel concept as applied to dc-dc converters for fault tolerant applications have also been presented [8]-[10]. Khan et al., for example, described a Fig.1. circuit of the multilevel dc-dc converter 303 P a g e
Vol.2, Issue.2, Mar-Apr 2012 pp-007-009 ISSN: 2249-6645 Fig. I shows the proposed H-bridge with two auxiliary leg and selector cells fault tolerant multi level dc-dc converter. The main H-bridge circuit i.e., devices S1-S4 and diodes D1-D4 is extended by four auxiliary switches (SA1/DA1, SA2/DA2, SA3/DA3 and SA4/DA4), three selector cells ( devices S5-S7, diodes D5-D16 and bidirectional switches SE1-SE3) and six additional bidirectional switches SE4-SE9 to form the multi level topology. Fault tolerant operation is achieved by using and controlling the PWM duty cycles of the individual switches to produce the required average output voltage with the minimum number of switches and diodes. The paper examines fault scenarios to demonstrate the full fault tolerant capacity of the proposed converter. Different of and duty cycles are evaluated. The variety of switching and PWM duty cycles provide fault tolerant operation. II. PROPOSED CONVERTER The operation of the H-bridge, dc-dc converter with resistive load under normal operating conditions is described in this section. TABLE I SWITCHING STATES FOR EACH VOLTAGE LEVEL: Voltage levels Current paths 30v D13, S7, D16, SE3, SE7, S4. 60v D9, S6, D12, SE2, SE7, S4. 90v D5,S5,D8,SE1,SE7,S4 120v S1, SE4, SE7, S4. 0v SE5, S2, SE7, S4. -30v S3, SE6, SE1, D6, S5, D7. -60v S3, SE6, SE2, D10, S6, D11. -90v S3, SE6, SE3, D14, S7, D15. -120v S3, SE6, SE5, S2. Fig.2. current path for conduction state corresponding to 30v output voltage. In the following analysis, only forward flow switch will be considered i.e., no negative voltage will be considered. Fig.3 shows four output voltage levels V Ln with PWM control at a fixed duty cycle D and a constant switching frequency. Assuming each voltage level is applied for an equal time T/4, the average output voltage V o can be calculated from V o = D M M N=1 V Ln (1) The proposed converter allows bidirectional flow and depending on the used, can produce nine output voltage levels (-120, -90, -60, -30, 0, 30, 60, 90, 120V) when operating without PWM control, as shown in Table I. The application of PWM control allows operation at any required average voltage between -120V and +120V. It should be noted here, that the circuit cannot achieve the redundancy needed for fault tolerant operation by varying the alone, each voltage level can be generated by only one switching combination as shown in Table I. The current path for the conduction state corresponding to an output voltage of 30V is shown in Fig. 2 as an example. Fig. 3. Output voltage levels. Where D is the duty cycle, m is the number of voltage levels and V Ln is the output voltage associated with level n. Equation (1) shows that the can produce a large number of possible output voltages when combined with all the possible values of converter duty cycles D. For example, Table II shows five possible with values of D to generate a 60V average output voltage. 304 P a g e
Vol.2, Issue.2, Mar-Apr 2012 pp-007-009 ISSN: 2249-6645 TABLE II POSSIBLE SWITCH COMBINATIONS TO GENERATE 60V AVERAGE OUTPUT State Output voltage PWM Average number level duty output 30v 90v 120v cycle D voltage 1 Yes Yes Yes 0.75 60v 2 Yes - Yes 0.80 60v 3 - Yes Yes 0.57 60v 4 - Yes - 0.67 60v 5 - - yes 0.50 60v Similarly Table III shows five possible with values of D to generate a 45V average output voltage. Fig.5. Measured output voltage waveform for V o =45V PWM duty cycle D=0.6 with the voltage levels of 30V and 120V respectively. TABLE III POSSIBLE SWITCH COMBINATIONS TO GENERATE 45V AVERAGE OUTPUT State number Output voltage level 30v 90v 120v PWM duty cycle D Average output voltage 1 Yes Yes Yes 0.5625 45v 2 Yes - Yes 0.6 45v 3 - Yes Yes 0.42 45v 4 - Yes - 0.5 45v 5 - - yes 0.375 45v The operation of the proposed converter was investigated using MATLAB/SIMULINK. Fig.6. Measured output voltage waveform for V o =60V; PWM duty cycle D=0.75 with the voltage levels of 30V, 90V and 120V respectively. Fig.4. Measured output voltage waveform for V o =60V PWM duty cycle D=0.8 with the voltage levels of 30V and 120V respectively. Here there are two output voltage levels but the average output voltage is 60v only. Fig.7. Measured output voltage waveform for V o =45V; PWM duty cycle D=0.5625 with the voltage levels of 30V, 90V and 120V respectively. 305 P a g e
Vol.2, Issue.2, Mar-Apr 2012 pp-007-009 ISSN: 2249-6645 proposed converter must demonstrate the ability to detect a short circuit or open circuit component fault and must change the appropriately to recover the required average output voltage. Here voltage and current sensors are used in order to sense the faults. Fig.8. Measured output voltage waveform for V o =60V; PWM duty cycle D=0.5 with the voltage levels of 120V respectively. Fig.10. fault tolerant multi level H-bridge dc-dc converter. Total number of sensors is low when compared with alternative circuit topologies [8]. However, the number sensors can be reduced even further by monitoring the output voltage using a neural network technique [12] or by using a smart IGBT gate drive with self-diagnosis and fault protection [13], the complete fault tolerant of the converter is shown in Fig Fig.9. Measured output voltage waveform for V o =45V; PWM duty cycle D=0.375 with the voltage levels of 120V respectively. In Fig 4-9 y-axis represents the voltage divisions and x-axis represents time divisions Figs. 4, 6, 8 show measured output voltages for three device switching (states 2, 1 and 5 in table II) to get 60V as average output voltage. Similarly Figs 5, 7, 9 show measured output voltages for three device switching (states 2, 1, 5 in table III) to get 45V as average output voltage. It is apparent from the figures that PWM control allows alternative switching options for the required output voltage level. Converter operation with the same switch was also simulated using PSpice showing good agreement with measurement. IV. OPEN CIRCUIT FAULTS If an open circuit fault occurs in any of the main switches S1-S4 or D1-D4, the extended additional leg1 must be activated. If the open circuit fault occurs in both main switches and also additional leg1 then activate additional leg2. The switching sequence following an open circuit fault in S1 is discussed here in detail as an example. Under normal operating conditions, S1 is switched ON and the controller receives a current measurement from C s4. If the controller does not receive this signal while S1 is still switched ON, the controller will flag this as an open circuit fault in S1. The controller now identifies a new switching state that needs to be activated, in this case switches SA1 and SE8, in order to provide the required voltage. Fig 10 shows how SA1 and SE8 are switched on to maintain normal operation at the same output voltage when an open circuit fault occurs in S1. After the fault, the current passing through S1 falls to zero, but load current continues to flow through SA1 and SE8. III. FAULT TOLERANT INVESTIGATION Fault that can occur in the switches may be open circuit or short circuit fault. In this section both open and short circuit faults are discussed and the fault tolerant behavior of the converter is evaluated using the 60V operating states discussed in section II as an example. Here only one fault can be occur at a time or two faults can be occur the 306 P a g e
Vol.2, Issue.2, Mar-Apr 2012 pp-007-009 ISSN: 2249-6645 fault, the sequence of need to change to operate the convert at 120V and a duty cycle of D=0.5. Fig.10. Output voltage waveform before and after an open circuit fault in main switch S1. Under this conditions i.e., switch S1 is open circuited, load current flowing through SA1 and SE8 controller receives a current measurement from C s8 if the open circuit fault occurs in SA1 then controller will identifies a new switching state that needs to be activated now switches SA3 and SE10 will be activated after the fault current passing through SA1 falls to zero and load current continues to flow through SA3and SE10 as shown in the Fig.11 Fig.12. Output voltage waveform when open circuit fault occurs in switch S5 Fig.12. shows measured output voltage waveform when open circuit fault occurs in S5; similar results are obtained when simulating the operation of the circuit under the same fault conditions. V. SHORT CIRCUIT FAULT Diode short circuit faults are detected using the voltage sensors circuits shown in Fig13 short circuit faults in the devices are detected via the gate drive circuits. On detection, short circuit faults are isolated by deactivating the corresponding selector switch. The control of the system is more complex when compared with open circuit faults responses due to the large number of voltage sensors and switches needed to detect and isolate each fault. The switching sequence following a short circuit fault in S1 is discussed here as an example. Fig.11. Output voltage waveform before and after an open circuit fault in main switch S1 and auxiliary switch SA1. If the open circuit fault occurs in any of the selector switching cells (devices S5-S7, diodes D5-D16 and bidirectional switches SE1-SE3), the converter will no longer be able to produce the required average output voltage using the existing switch state combination. For example, under normal operating conditions, the converter produces output voltage levels of 30, 90, 120 with duty cycle off D=0.75 to generate V o = 60V. If an open circuit fault were to occur in switch S5 (say), the converter is no longer able to produce a voltage level of 90V leading to the loss of the required 60V output voltage. On detecting the Fig.13. Output voltage waveform when short circuit fault occurs in selector switch S5 If the short circuit fault occurs in switch S1. On detecting the fault, SE4 is switched OFF and SE8 and SA1 switched on to initiate the new conduction state. Fig.13 show output voltage waveform generating 60V average output voltage and voltage after the fault is zero along switch S5. 307 P a g e
Vol.2, Issue.2, Mar-Apr 2012 pp-007-009 ISSN: 2249-6645 converter open circuit and short circuit fault. This converter also works even though fault occur in auxiliary leg VI. CONCLUSION Fault tolerant multilevel H-bridge dc-dc converter topology has been presented in this paper. Different are combined with PWM control to produce and maintain a constant average output voltage despite the occurrence of FAULT TOLERANT INVESTIGATIONS IN THE DIODES ACTIONS Switches Open circuit faults Short circuit faults D1 D2 D3 D4 DA1 DA2 D5 D6 D7 D8 D9 D10 D11 D12 D13 D14 Change to Change to Change to Change to Change to Deactivate SE4 and activate SA1 and SE8 Keep SE5 open Keep SE6 open Deactivate SE7 and activate SA2 and SE9 Deactivate SE8 and activate S1 and SE4 Deactivate SE9 and activate S4 and SE7 Deactivate SE1 and change to and D Deactivate SE2 and change to and D Deactivate SE3 APPENDIX TABLE VI D15 D16 Change to Switches S1 S2 S3 S4 SA1 SA2 S5 S6 S7 and change to and D Table VII FAULT TOLERANT INVESTIGATION IN POWER DEVICES Open circuit faults Activate SA1 and SE8 Not involved in forward Not involved in forward Activate SA2 and SE9 Activate S1 and SE4 Activate S4 and SE7 Change to Change to Change to Actions Short circuit faults Deactivate SE4 and activate SA1 and SE8 keep SE5 always open Keep SE6 always open Deactivate SE7 and activate SA2 and SE9 Deactivate SE8 and activate S1 and SE4 Deactivate SE9 and activate S4 and SE7 Deactivate SE1 and Change to Deactivate SE2 and Change to Deactivate SE3 and Change to 308 P a g e
Table VIII FAULT TOLERANT INVESTIGATION IN THE POWER DEVICES IF TWO SWITCHES FAILS switches Open circuit fault S1, SA1 Activate SA3 and SE10 S4, SA2 Activate SA4, SE 11 S3 S2 International Journal of Modern Engineering Research (IJMER) Vol.2, Issue.2, Mar-Apr 2012 pp-007-009 ISSN: 2249-6645 clamped dc/dc converter featuring fault tolerant capability, IEEE Trans. Power Electron., vol. 24, no. 1, pp. 14 24, Jan. 2009. [10] V. Choudhary, E. Ledezma, R. Ayyanar, and R.M. Short circuit Button, Fault tolerant circuit topology and control fault method for input-series and output-parallel modular Deactivate SE4 DC-DC converters, IEEE Trans. Power Electron., and activate vol. 23, no. 1, pp. 402 411, Jan. 2008. SA3 and SE10 [11] G. Ceglia,V. Guzman, C. Sanchez, F. Ibanez, Deactivate J.Walter, and M. I. Gimenez, A new simplified SE47 and multilevel inverter topology for DC-AC conversion, activate SA4 IEEE Trans. Power Electron., vol. 21, no. 5, pp. and SE11 1311 1319, Sep. 2006. Not involved Keep SE6 [12] S. Khomfoi and L. M. Tolbert, Fault diagnostic in forward always open system for a multilevel inverter using a neural flow network, IEEE Trans. Power Electron., vol. 22, no. operation 3, pp. 1062 1069, May 2007. Not involved keep SE5 [13] C. Lihua, F. Z. Peng, and C. Dong, A smart gate in forward always open drive with self-diagnosis for MOSFETs and flow IGBTs, in Proc. 23rd Annu. IEEE Appl. Power operation Electron. Conf. Expo. (APEC 2008), pp. 1602 1607. REFERENCE [1] M. R. Holman and S. J. Rowland, Design and development of a new cryosurgical instrument utilizing the Peltier thermoelectric effect, J. Med. Eng. Technol., vol. 21, no. 3 4, pp. 106 110,1997. [2] P. A. Kullstam, Availability, MTBF and MTTR For repairable M out of N system, IEEE Trans. Rel., vol. R-30, no. 4, pp. 393 394, Oct. 1981. [3] R. V.White and F.M.Miles, Principles of fault tolerance, in Proc. IEEE Appl. Power Electron. Conf. Expo. APEC, San Jose, Cost Rica, 1996, pp. 18 25. [4] A. Chen, L. Hu, L. Chen, Y. Deng, and X. He, A multilevel converter topology with fault-tolerant ability, IEEE Trans. Power Electron., vol. 20, no. 2, pp. 405 415, Mar. 2005. [5] X.Kou, K. A. Corzine, andy. L. Familiant, A unique fault-tolerant design for flying capacitor multilevel inverter, IEEE Trans. Power Electron., vol. 19, no. 4, pp. 979 987, Jul. 2004. [6] B. Francois and J. P. Hautier, Design of a fault tolerant control system for a N.P. C. multilevel inverter, in Proc. IEEE Int. Symp. Ind. Electron., L Aquila, Italy, 2002, vol. 4, pp. 1075 1080. [7] S.Miaosen, P. F. Zheng, and L.M. Tolbert, MultilevelDC-DC conversion system with multiple DC sources, IEEE Trans. Power Electron., vol. 23, no. 1, pp. 420 426, Jan. 2008. [8] K. Ambusaidi, V. Pickert, and B. Zahawi, Computer aided analysis of fault tolerant multilevel DC-DC converters, in Proc. IEEE Conf. Power Electron., Drives Energy Syst. Ind. Growth, New Delhi, India, 2006, pp.1 6. [9] F. H. Khan and L. M. Tolbert, Multiple load-source integration in a multilevel modular capacitor 309 P a g e